At present, the brake discs of military and commercial aircraft are normally made from carbon-carbon composites. Traditionally, C—C composites used as friction materials are produced by combining carbon fibers with a carbon matrix material which is deposited around the fibers using a Chemical Vapor Infiltration (CVI) process or a Chemical Vapor Deposition (CVD) process to provide the composites with the requisite density. CVI/CVD processing is an expensive, capital intensive, and time-consuming process, frequently taking several months to complete. Therefore, there is a need for improvements to the densification procedure in the manufacture of C—C composite friction materials. Such desirable improvements ideally include reduction in capital investment, improvements to the mechanical and thermal properties of the composites, and improvement to the friction and wear performance of the friction material (e.g., aircraft brake discs) made from the composites.
Background prior art with respect to nonwoven preform aspects of the present invention includes the following. EP 1 724 245 A1 describes a process for producing carbon-carbon composite preform, by: providing short carbon fiber or fiber precursor segments; providing particulate pitch; combining the fiber segments and pitch particles in a mold; subjecting the resulting mixture to elevated pressure to create an uncarbonized preform; placing the preform in a constraint fixture; and carbonizing the combined components in the constraint fixture at an elevated temperature to provide a preform having a desired density. US 2008/0090064 A1 discloses a carbon-carbon composite material comprising carbonized woven or nonwoven fabric-based preforms. A method taught in this document contemplates densifying the preform and subsequently adding a ceramic additive thereto in order to enhance the properties of the final product. US 2008/0041674 A1 discloses annular drive inserts which are placed within an annular opening within a brake disk. The annular drive inserts may comprise carbon-carbon composite which has been treated with antioxidant. U.S. Pat. No. 7,374,709 B2 describes a method in which specific end-use application friction requirements are satisfied by tailoring a level of carbon in a selected carbon/carbon preform, heat treating the carbon/carbon composite preform to affect thermal conductivity so as to optimize overall braking performance prior to ceramic processing, and by selecting an optimum level of ceramic hard phase to achieve satisfactory friction disc wear life and friction characteristics of a resulting braking material. Additional background patents and publications include: U.S. Pat. No. 7,252,499 B2; U.S. Pat. No. 7,172,408 B2; U.S. Pat. No. 7,025,913 B2; and U.S. Pat. No. 6,939,490 B2.
Background prior art with respect to the densification aspects of the present invention includes the following. US 2006/0279012 A1 discloses a carbon fiber preform densification by pitch infiltration wherein the pitch infiltration step may be facilitated by the application of vacuum and/or pressure. U.S. Pat. No. 4,318,955 discloses a method of making a carbon brake product wherein fibers are packed and then twice saturated with pyrocarbon, with a machining step therebetween, and heat treatment at 2000° C., to a final density of 1.75-1.8 g/cm. U.S. Pat. No. 6,077,464 discloses a method of making carbon-carbon composite materials which includes a variety of densification methods which may be used singularly or in various combinations. See e.g. column 4, lines 40-45. U.S. Pat. No. 6,342,171 B1 discloses a process of stabilizing a pitch-based carbon foam which includes densification of the foam with four cycles of combined VPI and PIC. See e.g. column 12, lines 8-40. US 2004/0105969 A1 discloses manufacture of carbon composites which includes densification of the preform by resin or pitch via vacuum and pressure.